This invention relates generally to imaging systems using pixilated detectors, and more particularly to pixilated semiconductor detectors in imaging systems.
Imaging devices, such as gamma cameras and computed tomography (CT) imaging systems, are used in the medical field to detect radioactive emission events emanating from a subject, such as a patient and to detect transmission x-rays not attenuated by the subject, respectively. An output, typically in the form of an image that graphically illustrates the distribution of the sources of the emissions within the object and/or the distribution of attenuation of the object is formed from these detections. An imaging device may have one or more detectors that detect the number of emissions, for example, gamma rays in the range of 140 keV, and may have one or more detectors to detect x-rays that have passed through the object. Each of the detected emissions and x-rays is typically referred to as a “count,” but the detected emissions may also be counted together as a ‘signal current’. The detector also determines the number of counts received at different spatial positions. The imager then uses the count tallies to determine the distribution of the gamma sources and x-ray attenuators, typically in the form of a graphical image having different colors or shadings that represent the processed count tallies.
A pixilated semiconductor detector, for example, fabricated from cadmium zinc telluride (CZT), may provide an economical method of detecting the gamma rays and x-rays. However, a low energy tail on the energy spectrum resulting from the CZT interaction with the radiation may interfere with the ability to distinguish direct gamma rays and x-rays from gamma rays and x-rays that have scattered in the subject. The tail may result in part from a different response of the semiconductor material in the regions between the pixels. Because of intermediate surface conductivity of the semiconductor between the pixel anodes, electrons arrive late to the anode, resulting in “ballistic deficit”. A low energy tail on the energy spectrum may also result from low hole mobility or trapping that causes charge integration derived from the pixel with respect to the common cathode to be incomplete.